This invention relates generally to air-activated dispensing systems for dispensing viscous materials such as glue or solder paste, and in particular to an air-activated dispensing system having a stepping plunger to eliminate the air buffer that develops as material is dispensed.
In manufacturing electronic equipment, one step in the process involves mounting electronic components on circuit boards, ceramic substrates, or other media. In some instances, a component is simply glued to a circuit board until the component leads are soldered, for example, by well-known wave soldering techniques. In other instances, a component may be soldered into place using paste solder dispensed onto a board at a particular location and applying localized heat. Accordingly, dispensing machines are provided which dispense viscous materials such as glue and solder paste onto the surface of a medium such as a printed circuit board at predetermined locations.
Typically, prior art dispensing systems are activated with pulses of air pressure applied via an air line to force a small amount, or "dot," of material from the nozzle of a syringe onto the circuit board. It is important that the dots be uniform in size and material so that problems are not created by having too much or too little material to attach components, and so that material is not wasted.
It can be seen that as more and more material is dispensed from the syringe, the space, or air buffer, inside the syringe becomes larger and larger, replacing the material as it is forced downward and dispensed. This creates a situation in which the dispensed dots get smaller because the air compresses in the air buffer, increasing the response time between the applied air-pressure pulse and movement by material so that material is not dispensed when it should be, and also reducing the dispensing force. In the prior art, attempts to overcome this problem have been less than satisfactory, and typically have involved increasing the air pressure or changing the timing of the air pulses as material is dispensed. However, it becomes increasingly difficult to control the pulse response time and dispensing force as the syringe nears empty.